Various kinds of alloys have been used as the material for medical devices such as stents and guide wires. For example, Japanese Application Publication No. 2003-325655 discloses a stent consisting of NiTi shape memory alloys and Japanese Application Publication No. 9-182799 discloses a guide wire consisting of NiTi-based alloys, CuZnAl-based alloys, CuAlNi-based alloys, or the like.
NiTi-based alloys, FeMnSi-based alloys, CuAlNi-based alloys, CuZnAl-based alloys, or the like are generally called a shape memory alloy, and among these, there is an alloy showing superelasticity at human body temperature (around 37° C.). The superelastic shape memory alloys have a crystal structure in which stress-induced type martensitic transformation is generated and have characteristics of returning to their original shape after releasing the stress. The term, “superelasticity” herein means that even if the material is deformed (bent, stretched, compressed, and twisted) at service temperature to the region in which ordinary metals undergo plastic deformation, releasing the deformation stress results in recovery to nearly the original shape before deformation without heating. The superelastic shape memory alloys having such characteristics can be used with advantage in stents, guide wires, or the like which are used in the living body.
Since the superelastic shape memory alloys are generally a “soft” alloy, there were some cases where the stress value in the plateau region (the region in which the stress value shows nearly constant value with respect to an increase of strain in the stress-strain curve) is insufficient depending on the application. In particular, in medical devices such as stents and guide wires which are arranged or used in the living body, more reduction in size is desired. However, when the stress value in the plateau region (the strength) is insufficient, it is difficult to promote a reduction in size such as a reduction in wall thickness and a reduction in diameter.